1. Field of the Invention
The present invention relates to an optical signal processor which can multiplex or demultiplex multiwavelength signal light.
2. Related Background Art
Diffraction grating devices act as so-called wavelength branching means. When multiplexed multiwavelength signal light is inputted, a diffraction grating device can cause the multiwavelength signal light to branch off spatially into the individual wavelengths. An optical signal processor using such wavelength branching means can receive multiplexed multiwavelength signal light, demultiplex it into individual wavelengths or bands, and output them; or multiplex multiwavelength signal light components inputted for the individual wavelengths or bands and output thus multiplexed signal light. The optical signal processor is used as an optical multiplexer, an optical demultiplexer, an optical ADM (Add Drop Multiplexer), and the like in optical communication systems.
Such an optical signal processor is disclosed, for example, in Document 1: U.S. Pat. No. 5,960,133. The optical signal processor disclosed in Document 1 comprises a reflection type diffraction grating device as wavelength branching means, a lens, and mirrors. The diffraction grating device causes the light fed into an input port to branch off spatially into individual wavelengths. The lens converges the individual wavelength components of signal light separated by the diffraction grating device in terms of wavelength. The mirrors are disposed at respective points on which the individual wavelength components of signal light are converged by the lens. The individual wavelength components of signal light reflected by the mirrors pass the lens and diffraction grating device in the direction opposite from the outgoing path, so as to be outputted from output ports. The mirrors are provided for the individual wavelengths, whereas the angles of inclination of their reflecting surfaces are controllable. By adjusting the inclination angle of the reflecting surface of each mirror, the optical signal processor acts as an optical ADM while being able to change the wavelength of signal light to be added or dropped.
The optical signal processor configured as mentioned above has a structure in which wavelength components of signal light after wavelength separation are converged onto reflecting surfaces of mirrors by a lens. Therefore, in order to improve the wavelength resolution, the degree of convergence is required to be high at the time when the signal light is converged by the lens. In general, for attaining a high degree of convergence, it will be sufficient if the beam diameter of light is made greater when the light is incident on the lens. However, in the optical signal processor configured as mentioned above, it is necessary for the individual wavelength components of signal light to be arranged parallel to each other between the input/output port and the diffraction grating device, so as to be kept from spatially overlapping each other. Therefore, the optical signal processor must enhance its size if its wavelength resolution is to be improved, whereby there is a tradeoff between the improvement in wavelength resolution and the smaller size.
In order to overcome the problem mentioned above, it is an object of the present invention to provide an optical signal processor which can improve its wavelength resolution and reduce its size.
The optical signal processor in accordance with the present invention comprises optical input/output means, a first optical system, wavelength branching means, a second optical system, and reflecting means. The optical input/output means includes a plurality of input/output ports for inputting or outputting light. The plurality of input/output ports have respective light input/output directions, in parallel with each other, located on a first virtual plane. The optical input/output means inputs light into any of the plurality of input/output ports and outputs the light from any of the other input/output ports. The first optical system collimates the light arriving from any of the plurality of input/output ports, and outputs thus collimated light. The wavelength branching means receives the light collimated by the first optical system, spatially separates the light in terms of wavelength, and outputs thus obtained wavelength light components. Thus outputted wavelength light components have respective optical axes located on a second virtual plane. The second optical system receives the wavelength light components outputted from the wavelength branching means after wavelength separation, and converges the wavelength light components. The reflecting means includes a mirror with a reflecting surface positioned at a light-converging point of the wavelength light components converged by the second optical system. The reflecting means causes the light reflected by the mirror to be outputted from any of the plurality of input/output ports by way of the second optical system, wavelength branching means, and first optical system. The first and second virtual planes are not parallel to each other. The light fed into the wavelength branching means after being collimated by the first optical system has a greater beam width in a direction parallel to the second virtual plane than in a direction perpendicular to the second virtual plane.
The optical signal processor is operable as an optical demultiplexer, an optical multiplexer, or an optical ADM. Namely, when multiplexed signal light is fed into any of the plurality of input/output ports included in the optical input/output means, the multiplexed signal light is collimated by the first optical system so as to be fed into the wavelength branching means, and is spatially separated in terms of wavelength by the wavelength branching means so as to be outputted into respective directions corresponding to the individual wavelengths. The wavelength components of signal light separated in terms of wavelength are converged by the second optical system so as to be made incident on and reflected by any mirror included in the reflecting means. Thus reflected wavelength components of signal light are outputted from any of the plurality of input/output ports after passing the second optical system, wavelength branching means, and first optical system. As such, the multiplexed signal light is demultiplexed. When the light advances in the opposite direction, multiwavelength signal light components are multiplexed, and thus multiplexed signal light is outputted.
Here, the plurality of input/output ports included in the optical input/output means have respective optical input/output directions, in parallel with each other, located on the first virtual plane. The wavelength components of light outputted from the wavelength branching means after wavelength separation have respective optical axes located on the second virtual plane. The first and second virtual planes are not parallel to each other. The light collimated by the first optical system so as to be fed into the wavelength branching means has a greater beam width in a direction parallel to the second virtual plane than in a direction perpendicular to the second virtual plane. Such characteristic features allow the optical signal processor to improve its wavelength resolution and reduce its size.
Preferably, the wavelength branching means includes a diffraction grating device. Preferably, the first and second virtual planes are perpendicular to each other. Preferably, a line connecting a point where an optical axis of light fed from the first optical system into the wavelength branching means intersects the wavelength branching means and a point where an optical axis of light fed from the second optical system into the wavelength branching means intersects the wavelength branching means is perpendicular to the second virtual plane.
Preferably, any two mirrors included in the reflecting means have respective inclination angles of reflecting surfaces different from each other about a line, parallel to the second virtual plane and perpendicular to an optical axis of the second optical system, passing the light-converging point. Preferably, each mirror included in the reflecting means has a reflecting surface with an inclination angle variable about a line, parallel to the second virtual plane and perpendicular to an optical axis of the second optical system, passing the light-converging point.
Preferably, the optical signal processor further comprises polarization separating means and polarization plane rotating means. The polarization separating means is disposed between the input/output means and the wavelength branching means. The polarization separating means separates the light fed into any of the plurality of input/output ports into respective polarized light components having first and second directions orthogonal to each other in terms of polarization, and outputs a first light beam of the polarized light component having the first direction and a second light beam of the polarized light component having the second direction. The polarization plane rotating means is disposed between the polarization separating means and the wavelength branching means. The polarization plane rotating means receives any of the first and second light beams outputted from the polarization separating means, rotates a polarization direction of the received light beam to make the first and second light beams have the same polarization direction yielding the highest wavelength branching efficiency in the wavelength branching means, and outputs thus rotated light beam. Preferably, a line connecting a point where an optical axis of the first light beam fed from the first optical system into the wavelength branching means intersects the wavelength branching means and a point where an optical axis of the second light beam fed from the first optical system into the wavelength branching means intersects the wavelength branching means is parallel to the second virtual plane. Preferably, a line connecting a point where an optical axis of the first light beam fed from the first optical system into the wavelength branching means intersects the wavelength branching means and a point where an optical axis of the second light beam fed from the second optical system into the wavelength branching means intersects the wavelength branching means is perpendicular to the second virtual plane. In this case, the loss upon demultiplexing or multiplexing is low, and the polarization-dependent loss is small.
Preferably, mirrors included in the reflecting means have respective reflecting surfaces with the same inclination angle about a line, perpendicular to the second virtual plane, passing the light-converging point.
Preferably, each mirror included in the reflecting means has a reflecting surface with an inclination angle variable in N stages, whereas the optical input/output means includes N+1 input/output ports (N being an integer of 2 or greater). This is preferable for realizing an optical demultiplexer or optical multiplexer.
Preferably, each mirror included in the reflecting means has a reflecting surface with an inclination angle variable in N stages, whereas the optical input/output means includes 2N input/output ports (N being an integer of 2 or greater). This is preferable for realizing an optical ADM.
Preferably, the optical input/output means comprises a common input port, a common output port, an n-th channel input port, and an n-th channel output port. Preferably, the n-th channel input port and n-th channel output port input or output signal light in the same channel. Preferably, the number of input/output ports located between the common input port and the n-th channel input port is identical to the number of input/output ports located between the common output port and the n-th channel output port.
Preferably, in the optical signal processor in accordance with the present invention, the common input port and the common output port are adjacent to each other, whereas the n-th channel input port and the n-th channel output port are adjacent to each other.
Preferably, in the optical signal processor in accordance with the present invention, a line of intersection between the reflecting surface of each mirror included in the reflecting means and a plane, parallel to the second virtual plane, including the light-converging point, is a curve in an area including a center position of the light-converging point. Preferably, the curve has a variable curvature.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings. They are given by way of illustration only, and thus should not be considered limitative of the present invention.